1. Field of the Invention
The invention relates in general to a liquid crystal display (LCD) and driving method thereof, and more particularly to a LCD applying a feed-through voltage and driving method thereof.
2. Description of the Related Art
In a conventional LCD, if the sub-pixels of a display unit have the same area, when receiving the same data voltage, each sub-pixel has substantially the same capacitance of the liquid crystal capacitor, gate-drain parasitic capacitor and storage capacitor, and thus the same feed-through voltage. Owing to the fact that the capacitance of the liquid crystal capacitor of a sub-pixel is related to the data voltage received and the area and shape of the sub-pixel, when two sub-pixels are different in area or shape, the two sub-pixels will have different feed-through voltages.
Referring to FIG. 1, an equivalent circuit diagram of a part of the sub-pixels in a conventional LCD is shown. In a driving circuit of sub-pixels, it is supposed that a sub-pixel A and a sub-pixel B are different in either area or shape. Therefore, when the sub-pixels A and B receive the same data voltage, the liquid crystal capacitor Clc1 of the sub-pixel A is different from the liquid crystal capacitor Clc2 of the sub-pixel B. Because the sub-pixels A and B have the same transistor design, the gate-drain parasitic capacitor Cgd1 of the sub-pixel A is the same as the gate-drain parasitic capacitor Cgd2 of the sub-pixel B and the storage capacitor Cs1 of the sub-pixel A is also the same as the storage capacitor Cs2 of the sub-pixel B.
The feed-through voltage will be disclosed in detail according to the equivalent circuit of the sub-pixel A in FIG. 1. Referring to FIG. 1, at the time when the voltage at the gate line changes, for the sake of capacitor coupling of the transistor switch T1, the voltage of the pixel electrode of the sub-pixel A is shifted down due to a feed-through effect and the shift amount of the pixel-electrode voltage is called a feed-through voltage. When the gate line is enabled, the gate voltage of the sub-pixel A is increased from Vgl to Vgh, the feed-through voltage is (Vgh−Vgl)×Cgd1/(Cs1+Cgd1+Clc1). It can be seen that the feed-through voltage is related to the liquid crystal capacitor, gate-drain parasitic capacitor and storage capacitor. Therefore, when the sub-pixel A is different from the sub-pixel B in area, the liquid crystal capacitor, gate-drain parasitic capacitor and storage capacitor of the sub-pixel A are also respectively different from those of the sub-pixel B. As a result, the feed-through voltages generated by the sub-pixels A and B are also different. Therefore, when a positive data voltage or a negative data voltage corresponding to the same grey value, is input, the sub-pixels A and B will generate different luminance.
Referring to FIG. 2, an example of a waveform diagram of the driven sub-pixels A and B is shown. The vertical axis (ordinate) of the waveform diagram represents a voltage value and the transverse axis (abscissa) of the waveform diagram represents time. The waveform 201 is a partial waveform of a scan-line signal and the waveform 202 is a partial waveform of a voltage inputted to the sub-pixels A and B via a data line. The waveforms 203 and 204 are respectively voltage waveforms of the pixel electrodes of the sub-pixels A and B. From the waveform 203, it can be seen that in the first frame period F1, after the sub-pixel A receives a positive data voltage V+, due to the feed-through effect generated as the scan voltage is decreased from a high level to a low level, the voltage of the pixel electrode of the sub-pixel A will be shifted down by a first feed-through voltage ΔVf1 to become a voltage Va+. In the first frame period F1, after the sub-pixel B receives the same positive data voltage V+, due to the feed-through effect, the voltage of the pixel electrode of the sub-pixel B will be shifted down by a second feed-through voltage ΔVf2 to become Vb+.
Similarly, in the second frame period F2, after the sub-pixel A receives a negative data voltage V−, due to the feed-through effect generated as the scan voltage is decreased from a high level to a low level, the voltage of the pixel electrode of the sub-pixel A will be shifted down by a first feed-through voltage ΔVf1 to become a voltage Va−. In the second frame period F2, after the sub-pixel B receives the same negative data voltage V−, due to the feed-through effect, the voltage of the pixel electrode of the sub-pixel B will be shifted down by a second feed-through voltage ΔVf2 to become Vb−.
Owing to the fact that the sub-pixels A and B are different in area, the first feed-through voltage ΔVf1 is not equal to the second feed-through voltage ΔVf2. It is assumed that ΔVa1 is an absolute difference between the voltage Va+ and the common voltage Vcom, ΔVb1 is an absolute difference between the voltage Va−and the common voltage Vcom, ΔVa2 is an absolute difference between the voltage Vb+ and the common voltage Vcom and ΔVb2 is an absolute difference between the voltage Vb− and the common voltage Vcom. When adjusting the positive data voltage and negative data voltage for driving the sub-pixels according to the first feed-through voltage ΔVf1 of the sub-pixel A, such that the positive pixel voltage and negative pixel voltage of the pixel electrode of the sub-pixel A are symmetrical to the common voltage Vcom under the feed-through effect, after the sub-pixel B receives the adjusted positive data voltage and negative data voltage, the voltage of the pixel electrode of the sub-pixel B is always not symmetrical to the common voltage Vcom under the feed-through effect. Therefore, when in polarity inversion, the sub-pixel B receives the positive data voltage and negative data voltage corresponding to the same grey value, due to the feed-through effect, the positive pixel voltage and negative pixel voltage of the pixel electrode of the sub-pixel B are not symmetrical with respect to the common voltage Vcom, and consequently, the sub-pixel B correspondingly displays different luminance, which results in frame flash.